The most common standard for mobile communications in the world is the Global System for Mobile telecommunications (GSM). In one specific implementation, GSM utilizes two bands of 25 MHz, which have been set aside for system use. The 890–915 MHz band is used for subscriber to base station transmissions (reverse link), and the 935–960 MHz band is used for base station to subscriber transmissions (forward link). The GSM protocol uses frequency division duplexing and time division multiple access (TDMA) techniques to provide base stations with simultaneous access to multiple users. Transmissions on both the forward and reverse link are made at a channel data rate of 270.833333 Kbps, using binary Gaussian minimum shift key (GMSK) modulation.
In the GSM protocol, there are traffic channels and control channels. The traffic channels carry the digitized voice or user data. One of the control channels is what is known as the frequency correction channel (FCCH), which is a special data burst which occupies time slot 0 for the very first GSM frame and is repeated every ten frames within a control channel multiframe. The FCCH burst allows each mobile station to synchronize its internal frequency standard (local oscillator) to the exact frequency of the base station.
The data burst carried by the FCCH is nominally at a frequency one-quarter of the channel data rate, i.e., 270.833333÷4 or 67.708 KHz. Thus, the frequency correction channel is a single tone at the nominal frequency of 67.708 KHz. However, because of various factors, such as co-channel interference, fading, and Gaussian noise, the frequency of the received FCCH tone may vary from the nominal 67.708 KHz. In order for the mobile station to operate optimally, it is important to precisely determine the frequency of the FCCH tone to within 100 Hz.
One prior art method of estimating the frequency of the FCCH tone is disclosed in my U.S. Pat. No. 5,761,250 entitled “Iterative Filtering Frequency Estimator and Estimation Method”. In this method, the FCCH burst is iteratively filtered to determine a pole estimate. Using the pole estimate, the frequency of the FCCH burst can be estimated. Still, the accuracy of the frequency estimation using this technique may not be sufficient.